In the generation of continuous pulse trains, such as the rectangular-wave signal which is commonly used to synchronize the components (e.g., flip-flops, gates, input-output peripherals) of digital computers, wide use is made of sine-wave oscillators followed by clipping and/or other pulse-shaping circuits. Frequently, in order to achieve a high order of stability without costly complications, crystals are incorporated in the circuitry to establish the frequency of oscillation. However, crystal oscillators inherently produce output signals rich in harmonic content. Presuming that what is desired is that the circuit be permitted to issue, for instance, its fundamental frequency only, it is required that there be provision for insuring that it "lock" to the fundamental to the exclusion of all the harmonics. Several ways of accomplishing this are known. Firstly, there may be used a crystal with a lower impedance at the desired harmonic in an oscillator circuit in which that impedance is a critical factor; the disadvantage here is that special precautions must be taken to insure that the crystal has the correct properties and close constraints thereon make it difficult to use production-quality units. Alternatively, additional frequency sensitive networks may be added to the oscillator loop, thereby, usually, adding a large phase shift at any frequency other than that desired; this requires that the rest of the oscillator loop be sensitive to the added phase shift, and may require the inclusion of special provisions to insure that the added phase shift inhibits oscillation. Also the additional frequency sensitive networks are often an inherent part of an amplifier, thus effectively limiting the oscillation frequency to an upper bound; a problem with this approach is that it is difficult to guarantee the upper frequency characteristics of an amplifier unless control is a function of the values of passive components instead of the limitations of active devices. This requires active devices with upper frequency limits certified to be higher than the frequency cutoff of the amplifier, a requirement not always readily accomplished.
If a crystal oscillator is considered in the time domain instead of the frequency domain, i.e., if bounds (upper and/or lower limits) are placed on the period of the output the use of one-shot timers and appropriate gates suggests several advantages. Firstly, circuit structure and operation are ideal for understanding by logicians of the computer field; analog amplifiers, filters, etc. need not be investigated or constructed. Secondly, the components are functionally the same as used in the rest of the logic system; thus, there is no need for special packaging for the clock oscillator circuits, for example. Thirdly, correct harmonic lock-on is assured by the one-shots; there are no phase margins or gain margins to calculate or compensations for any unexpected circuit parameter changes to include in the design.